1. Field of the Invention
The invention concerns a method for adjustment of radio-frequency pulses which are emitted by a radio-frequency antenna of a magnetic resonance measurement system in a magnetic resonance measurement. Moreover, the invention concerns a magnetic resonance measurement system with a radio-frequency antenna and with a corresponding adjustment device for adjustment of the field strength of radio-frequency pulses that are emitted by the radio-frequency antenna in a magnetic resonance measurement.
2. Description of the Prior Art
Magnetic resonance tomography has become a widespread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or the body part of the patient to be examined must initially be exposed to an optimally homogenous static basic magnetic field (generally designated as a B0 field), which is generated by a basic field magnet of the magnetic resonance measurement device. During the acquisition of the magnetic resonance images, rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding. With one or more radio-frequency antennas, radio-frequency pulses of a defined field strength are radiated into the examination subject. The magnetic flux density of these radio-frequency pulses is typically designated B1. The pulse-shaped radio-frequency field is therefore generally called a B1 field for short. By means of these radio-frequency pulses the nuclear spins of the atoms in the examination subject are excited such that they are moved from their state of equilibrium by an “excitation flip angle” (also called “flip angle” for short in the following) parallel to the basic magnetic field B0. The nuclear spins then precess in the direction of the basic magnetic field B0. The magnetic resonance signals thereby generated are acquired by radio-frequency receiving antennas. The receiving antennas can be either the same antennas with which the radio-frequency pulses are also radiated or separate receiving antennas. The magnetic resonance images of the examination subject are ultimately created based on the received magnetic resonance signals. Every image point in the magnetic resonance image is thereby associated with a small body volume, known as a “voxel”, and every brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal acquired from this voxel.
Before the actual MR imaging, patient-specific adjustments must be implemented in order to determine the system variables dependent on the measurement subject. Adjustment of the field strength of the radio-frequency pulses (i.e. of the RF pulse amplitudes) is one of these patient-specific adjustment. The fact is taken into account that the transmission antenna is attenuated dependent on the examination subject and the pulse amplitudes of the RF power amplifier that are necessary to achieve the desired B1 fields or flip angles in the examination subject consequently vary dependent on the subject. Given a homogeneous flip angle distribution within the subject, a clear relation exists between pulse amplitude of the radio-frequency pulse and the amplitude of the of the B1 field (and therewith also the achieved flip angle α) according to the equation
                              α          =                                    ∫                              t                =                0                            τ                        ⁢                          γ              ·                                                B                  1                                ⁡                                  (                  t                  )                                            ·                                                          ⁢                              ⅆ                t                                                    ,                            (        1        )            which generally follows a linear function. γ is the gyromagnetic ratio which can be considered as a fixed material constant for most magnetic resonance examinations, and τ is the effective duration of the radio-frequency pulse.
It cannot be assumed, however, that the RF field within the examination subject is homogenous due to the following influencing factors:                Due to the interaction of the RF field with the human body, a homogeneous B1 distribution cannot be assumed, because of the differing dielectric and electrical properties of the various tissue types. Instead, a strong variation of the B1 amplitudes and phases exists over the entirety of the body volume.        The finite extent of the antenna elements generating the RF pulses results in the existence of a sufficient B1 homogeneity only within a limited volume. This spatial dependency is, however, only weakly developed in comparison to the aforementioned body inhomogeneity and is for the most part monotonous in its radial dependency. In practice, it therefore represents only a subordinate problem for the pulse calibration, in particular at high field strengths.        
For the aforementioned reasons, a radio-frequency pulse with a defined pulse amplitude consequently does not lead to a fixed, defined flip angle over the entire excited volume, but rather to a multitude of generated flip angle values. The flip angle distribution is thereby generally a function of the electrical and dielectric properties of the subject and their geometric distribution. This flip angle variation over the considered volume inevitably leads to an ambiguity in the pulse calibration.
For example, given imaging of the abdomen, regions with strongly reduced B1 amplitude are frequently observed in the body center, in particular at high field strengths. However, regions with strongly increased B1 amplitude also simultaneously exist. If the flip angle required by a measurement sequence is regionally not achieved or is exceeded, this leads to a limited image quality in these regions, for example only a weak signal strength and a low contrast are produced.
In a conventional, known adjustment method, the ambiguity is partially compensated by a complex averaging over portions of the signal-emitting volume. For this purpose, an adjustment of the pulse amplitude ensues, for example with the following MR experiment with the pulse sequence schematically shown in FIG. 1:
A pulse sequence with three RF pulses (with the desired flip angles αS, 2αS, αS) is used, with a primary echo signal SE generated by this pulse sequence and a stimulated echo signal STE being evaluated.
During the RF excitation and the signal acquisition, a constant gradient field Gz is simultaneously present in the z-direction (normally the direction of the basic magnetic field B0) of the scanner such that a planar two-dimensional slice is centrally excited in the human body. The acquired signal is the spatially integrated signal from the entire slice volume.
After a Fourier transformation of both echo signals SE, STE, the frequency portions SSE and SSTE of the primary echo signal SE and of the stimulated echo signal STE (i.e. the amplitudes of the signals at the central frequency f=0 of the spectrum) are drawn upon for the flip angle calculation. The result is a “median” flip angle α existing in the central slice, which “median” flip angle α was achieved given an applied pulse amplitude, whereby the signal amplitudes SSE, SSTE multiplied with the B1 phase are inherently complexly averaged:
                              cos          ⁢                                          ⁢          α                =                                                            S                SE                            ·                              S                STE                                                                                                      S                  SE                                                            2                                ·                      e                                          Δ                ⁢                                                                  ⁢                T                                            T                ⁢                                                                  ⁢                1                                                                        (        2        )            wherein T1 designates the average relaxation time of the entire signal-emitting tissue and ΔT designates the interval between the second RF pulse and the third RF pulse of the exciting pulse sequence.
From this, a new pulse amplitude can be determined that is necessary in order to achieve a desired B1 field. The pulse amplitude so determined then can be verified by a new measurement and, if necessary, be adapted again. This is in particular necessary since, due to the complex averaging in which the spatially-dependent magnetization contributing to the respective echo is complex (thus added up according to magnitude and phase, the correlation between the pulse amplitude and the calculated flip angle will no longer be linear, but rather will saturate at high pulse amplitudes and possibly even lose its monotonous characteristic.
In addition to the cited non-linearity, one disadvantage of this method is the fact that the optimization of the pulse amplitude is based on a flip angle that is complexly averaged over the entire volume. If the area relevant for the imaging then lies in regions with relatively high or low B1 amplitude, the nominal flip angle required by the sequence does not coincide with the actual flip angle, which leads to limitations in the image quality.
As an alternative, two-dimensional, spatially-resolved methods also exist for B1 magnitude and phase determination in the framework of an adjustment. A spatially-specific pulse amplitude calibration can therewith ensue. The monotonous or linear correlation between the pulse amplitude and the measured B1 field is then reestablished for sufficiently small regions. The regions taken into account for the pulse calibration then correspond to, for example, the considered regions in the framework of the clinical imaging.
However, one disadvantage of this method is the long (in comparison to the previously described integrative method) measurement time that is dependent the necessary sequence of N phase coding steps, N designating the matrix size in the phase coding direction. A further disadvantage is in the higher sensitivity to movements of the examination subject (i.e. of the human body) during the measurement time.